Make GADT constructors adhere to the forall-or-nothing rule properly

Issue #18191 revealed that the types of GADT constructors don't quite
adhere to the `forall`-or-nothing rule. This patch serves to clean up
this sad state of affairs somewhat. The main change is not in the
code itself, but in the documentation, as this patch introduces two
sections to the GHC User's Guide:

* A "Formal syntax for GADTs" section that presents a BNF-style
  grammar for what is and isn't allowed in GADT constructor types.
  This mostly exists to codify GHC's existing behavior, but it also
  imposes a new restriction that addresses #18191: the outermost
  `forall` and/or context in a GADT constructor is not allowed to be
  surrounded by parentheses. Doing so would make these
  `forall`s/contexts nested, and GADTs do not support nested
  `forall`s/contexts at present.

* A "`forall`-or-nothing rule" section that describes exactly what
  the `forall`-or-nothing rule is all about. Surprisingly, there was
  no mention of this anywhere in the User's Guide up until now!

To adhere the new specification in the "Formal syntax for GADTs"
section of the User's Guide, the following code changes were made:

* A new function, `GHC.Hs.Type.splitLHsGADTPrefixTy`, was introduced.
  This is very much like `splitLHsSigmaTy`, except that it avoids
  splitting apart any parentheses, which can be syntactically
  significant for GADT types. See
  `Note [No nested foralls or contexts in GADT constructors]` in
  `GHC.Hs.Type`.

* `ConDeclGADTPrefixPs`, an extension constructor for `XConDecl`, was
  introduced so that `GHC.Parser.PostProcess.mkGadtDecl` can return
  it when given a prefix GADT constructor. Unlike `ConDeclGADT`,
  `ConDeclGADTPrefixPs` does not split the GADT type into its argument
  and result types, as this cannot be done until after the type is
  renamed (see `Note [GADT abstract syntax]` in `GHC.Hs.Decls` for why
  this is the case).

* `GHC.Renamer.Module.rnConDecl` now has an additional case for
  `ConDeclGADTPrefixPs` that (1) splits apart the full `LHsType` into
  its `forall`s, context, argument types, and result type, and
  (2) checks for nested `forall`s/contexts. Step (2) used to be
  performed the typechecker (in `GHC.Tc.TyCl.badDataConTyCon`) rather
  than the renamer, but now the relevant code from the typechecker
  can simply be deleted.

  One nice side effect of this change is that we are able to give a
  more accurate error message for GADT constructors that use visible
  dependent quantification (e.g., `MkFoo :: forall a -> a -> Foo a`),
  which improves the stderr in the `T16326_Fail6` test case.

Fixes #18191. Bumps the Haddock submodule.

2 files changed, 36 insertions, 40 deletions

diff --git a/compiler/GHC/Parser/PostProcess.hs b/compiler/GHC/Parser/PostProcess.hs
index 273fa0d704..645f56fc54 100644
--- a/compiler/GHC/Parser/PostProcess.hs
+++ b/compiler/GHC/Parser/PostProcess.hs
@@ -685,43 +685,35 @@ mkConDeclH98 name mb_forall mb_cxt args
                , con_args   = args
                , con_doc    = Nothing }
 
+-- | Construct a GADT-style data constructor from the constructor names and
+-- their type. This will return different AST forms for record syntax
+-- constructors and prefix constructors, as the latter must be handled
+-- specially in the renamer. See @Note [GADT abstract syntax]@ in
+-- "GHC.Hs.Decls" for the full story.
 mkGadtDecl :: [Located RdrName]
-           -> LHsType GhcPs     -- Always a HsForAllTy
-           -> (ConDecl GhcPs, [AddAnn])
+           -> LHsType GhcPs
+           -> ConDecl GhcPs
 mkGadtDecl names ty
-  = (ConDeclGADT { con_g_ext  = noExtField
-                 , con_names  = names
-                 , con_forall = L l $ isLHsForAllTy ty'
-                 , con_qvars  = tvs
-                 , con_mb_cxt = mcxt
-                 , con_args   = args
-                 , con_res_ty = res_ty
-                 , con_doc    = Nothing }
-    , anns1 ++ anns2)
+  | Just (mtvs, mcxt, args, res_ty) <- mb_record_gadt ty
+  = ConDeclGADT { con_g_ext  = noExtField
+                , con_names  = names
+                , con_forall = L (getLoc ty) $ isJust mtvs
+                , con_qvars  = fromMaybe [] mtvs
+                , con_mb_cxt = mcxt
+                , con_args   = args
+                , con_res_ty = res_ty
+                , con_doc    = Nothing }
+  | otherwise
+  = XConDecl $ ConDeclGADTPrefixPs { con_gp_names = names
+                                   , con_gp_ty    = mkLHsSigType ty
+                                   , con_gp_doc   = Nothing }
   where
-    (ty'@(L l _),anns1) = peel_parens ty []
-    (tvs, rho) = splitLHsForAllTyInvis ty'
-    (mcxt, tau, anns2) = split_rho rho []
-
-    split_rho (L _ (HsQualTy { hst_ctxt = cxt, hst_body = tau })) ann
-      = (Just cxt, tau, ann)
-    split_rho (L l (HsParTy _ ty)) ann
-      = split_rho ty (ann++mkParensApiAnn l)
-    split_rho tau                  ann
-      = (Nothing, tau, ann)
-
-    (args, res_ty) = split_tau tau
-
-    -- See Note [GADT abstract syntax] in GHC.Hs.Decls
-    split_tau (L _ (HsFunTy _ (L loc (HsRecTy _ rf)) res_ty))
-      = (RecCon (L loc rf), res_ty)
-    split_tau tau
-      = (PrefixCon [], tau)
-
-    peel_parens (L l (HsParTy _ ty)) ann = peel_parens ty
-                                                       (ann++mkParensApiAnn l)
-    peel_parens ty                   ann = (ty, ann)
-
+    mb_record_gadt ty
+      | (mtvs, mcxt, body_ty) <- splitLHsGADTPrefixTy ty
+      , L _ (HsFunTy _ (L loc (HsRecTy _ rf)) res_ty) <- body_ty
+      = Just (mtvs, mcxt, RecCon (L loc rf), res_ty)
+      | otherwise
+      = Nothing
 
 setRdrNameSpace :: RdrName -> NameSpace -> RdrName
 -- ^ This rather gruesome function is used mainly by the parser.
diff --git a/compiler/GHC/Parser/PostProcess/Haddock.hs b/compiler/GHC/Parser/PostProcess/Haddock.hs
index f232113c2e..409b0c120f 100644
--- a/compiler/GHC/Parser/PostProcess/Haddock.hs
+++ b/compiler/GHC/Parser/PostProcess/Haddock.hs
@@ -12,24 +12,28 @@ import Control.Monad
 -- -----------------------------------------------------------------------------
 -- Adding documentation to record fields (used in parsing).
 
-addFieldDoc :: LConDeclField a -> Maybe LHsDocString -> LConDeclField a
+addFieldDoc :: LConDeclField GhcPs -> Maybe LHsDocString -> LConDeclField GhcPs
 addFieldDoc (L l fld) doc
   = L l (fld { cd_fld_doc = cd_fld_doc fld `mplus` doc })
 
-addFieldDocs :: [LConDeclField a] -> Maybe LHsDocString -> [LConDeclField a]
+addFieldDocs :: [LConDeclField GhcPs] -> Maybe LHsDocString -> [LConDeclField GhcPs]
 addFieldDocs [] _ = []
 addFieldDocs (x:xs) doc = addFieldDoc x doc : xs
 
 
-addConDoc :: LConDecl a -> Maybe LHsDocString -> LConDecl a
+addConDoc :: LConDecl GhcPs -> Maybe LHsDocString -> LConDecl GhcPs
 addConDoc decl    Nothing = decl
-addConDoc (L p c) doc     = L p ( c { con_doc = con_doc c `mplus` doc } )
+addConDoc (L p c) doc     = L p $ case c of
+  ConDeclH98  { con_doc = old_doc } -> c { con_doc = old_doc `mplus` doc }
+  ConDeclGADT { con_doc = old_doc } -> c { con_doc = old_doc `mplus` doc }
+  XConDecl x@(ConDeclGADTPrefixPs { con_gp_doc = old_doc }) ->
+    XConDecl (x { con_gp_doc = old_doc `mplus` doc })
 
-addConDocs :: [LConDecl a] -> Maybe LHsDocString -> [LConDecl a]
+addConDocs :: [LConDecl GhcPs] -> Maybe LHsDocString -> [LConDecl GhcPs]
 addConDocs [] _ = []
 addConDocs [x] doc = [addConDoc x doc]
 addConDocs (x:xs) doc = x : addConDocs xs doc
 
-addConDocFirst :: [LConDecl a] -> Maybe LHsDocString -> [LConDecl a]
+addConDocFirst :: [LConDecl GhcPs] -> Maybe LHsDocString -> [LConDecl GhcPs]
 addConDocFirst [] _ = []
 addConDocFirst (x:xs) doc = addConDoc x doc : xs